output.md

---
- S
-
  - S
  -
    - S
    - a
  -
    - S
    - a
  - +
-
  - S
  - a
- '*'

parsetree.pl

use 5.010;
use strict;
use warnings;

use Marpa::R2;

my $g = Marpa::R2::Scanless::G->new( { 
    source => \(<<'END_OF_SOURCE'),
:default ::= action => [ name, value]
lexeme default = action => [ value ] latm => 1
        
    S ::= S S '+' | S S '*' | 'a'

:discard ~ whitespace
whitespace ~ [\s]+
END_OF_SOURCE
} );

my $input = <<EOI;
aa+a*
EOI

use YAML qw{ Dump };
say Dump ${ $g->parse( \$input ) };